1. Field of the Invention
The present invention relates to an optical scanning device in an image forming apparatus.
2. Description of the Related Art
Recently, cost reduction and high stability of image quality against temperature change are demanded of image forming apparatuses such as optical printers, digital copiers, and optical plotters.
Degradation of the image quality due to the temperature change is mainly caused by fluctuation of a beam waist position in a main-scanning direction and/or a sub-scanning direction arising from the temperature change. A change in volume of an optical element due to the temperature change and a change in oscillation wavelength of a laser diode due to the temperature change cause the fluctuation of the beam waist position in the main-scanning direction and/or the sub-scanning direction.
The change in oscillation wavelength of the laser diode is not always caused by the temperature change. It is known that the oscillation wavelength changes even when the temperature does not change. For example, chromatic aberration of an optical system used in an optical scanning device causes characteristic change. Therefore, high stability of image quality against the characteristic change is also demanded.
Japanese Patent Application Laid-open No. 2007-241182 proposes using a diffractive optical element that includes a multi-step diffractive surface, as an effective technology against degradation of image quality caused by the change in volume of an optical element and by the change in oscillation wavelength due to the temperature change and/or against degradation of image quality arising from the change in oscillation wavelength caused by any factor other than the temperature change. The diffractive surface is formed with a first surface having a diffraction effect and a second surface having a refraction effect which are joined to each other, and has characteristics that the power of the first surface and the power of the second surface are mutually cancelled out. By using the diffractive optical element, not only degradation of image quality due to the temperature change but also minimization of degradation of image quality due to the change in oscillation wavelength caused by any factor other than the temperature change can be achieved.
However, in a laser scanning device disclosed in Japanese Patent Application Laid-open No. 2007-241182, because the diffractive optical element having the multi-step diffractive surface is made of resin, a linear expansion coefficient of the diffractive optical element itself is 7.0×10−5/K, which is comparatively large. Therefore, if a wide range of temperature change is assumed, a phase function coefficient in the diffractive optical element required to correct the fluctuation of the beam waist position in the main-scanning direction and/or the sub-scanning direction needs to be large, that is, the diffraction power needs to be increased. Besides, because the step of the multi-step diffractive surface needs to be increased, this causes machining to become difficult, and the diffractive surface may become susceptible to wavelength change which is independent of the temperature change.